Low-drag fairing configuration for flexible towing cables

ABSTRACT

A cable fairing has a flexible hollow nose and a rigid tail; the fairing is in short sections with each adjacent pair connected by flexible pivotal links.

United States Patent [72] Inventors Neville E. Hale Port Credit,Ontario; Kenneth Gardner, Mimico, Ontario, both of Canada [21 I Appl.No. 875,864

[22] Filed Nov. 12, 1969 [45] Patented Oct. 12,1971

[73} Assignee Fathom Oceanology Limited Port Credit, Ontario, Canada[32] Priority Nov. 23, 1968 [3 3 1 Great Britain [54] LOW-DRAG FAIRINGCONFIGURATION FOR FLEXIBLE TOWING CABLES 7 Claims, 5 Drawing Figs.

52 us. c|.. 114/2351 [51] Int.Cl B63b 21/00 [50] Field of Search114/235, 235.2 F

[56] References Cited UNITED STATES PATENTS 2,397,957 4/1946 Freeman114/2352 F 3,086,491 4/1963 Rather et al. 1 14/2352 F 3,194,204 7/1965Nichols et a1. 114/2352 F 3,443,020 5/1969 Loshigian 1 14/2352 FXPrimary Examiner-Trygve M. Blix Attorney-Douglas S. Johnson ABSTRACT: Acable fairing has a flexible hollow nose and a rigid tail; the fairingis in short sections with each adjacent pair connected by flexiblepivotal links.

PATENTED 0m 1 21% 13,61 1. 976

sum 2 or 2 LOW-DRAG FAIRING CONFIGURATION FOR FLEXIBLE TOWING CABLES IFIELD OF THE INVENTION The invention relates to cable fairings.

DESCRIPTION OF PRIOR ART In the application of cables for underwatertowing such as variable depth sonar, water sampling, hydrographicsurveys andrelated oceanographic fields, fairings of aerofoilconfiguration are attached to said cable in order to reduce hydrodynamicdrag and vibration.

In theory, a continuous fairing moulded to the cable is ideal. Inpractice, however, it is found that as the cable takes on acharacteristic catenary curvature due to hydrodynamic drag actingthereon, the trailing edge of the fairing is thrown into tension and theleading edge is thrown into compression. In consequence, the fairingtries to equalize these stresses by ruddering to one side or the other.The result is the develop ment of instability in the cable system.

A solution to thisproblem now widely used is a sectionalized fairingwhich comprises a trailing plastic member joined to a metallic nosepiece which surrounds the cable. Such a fairing has, dependent uponshape, a theoretical drag coefficient of about 0.06 as compared to abare cable drag coefficient of 1.2. In practice, however, it is foundthat the actual drag is much higher than this.

It has been established by carefully controlled hydrodynamic tests thatthe majority of the excess drag comes about by the necessity of a Vnotch which is formed between the butting adjacent nose sections. ThisV' notch has to exist with rigid nose fairings to allow the fairing tofollow the curvature of the cable without interference. The notch has tobe quite liberal in order for the cable to be wound on a storage drumand to pass over towing sheaves.

Somewhat lesser contributors to drag occur due to discontinuities acrossthe flow lines at the nose-tail interface and at rivets that join thecomponents together.

The development of an improved fairing, therefore, is not so much aquestion of selecting a more perfect sectional profile but rather theevolution of a design that eliminates the causes of departure fromtheoretical results.

The principal object of our invention therefore is to provide a fairingthat is flexible in the nose section thereof in order that it maytemporarily yield in compression at the abutment with its adjacentsection as the cable bends and regains its original dimensions when thecable is straight, thus eliminating the V notch discontinuity.

A second important object is to provide a fairing that combines axialrigidity in the tail portion of the fairing with the flexible noseportion in order that hydrodynamic forces acting parallel to the axis ofthe cable, particularly in high-speed applications, do not distort thefairing.

A third object is to provide a fairing design and manufactur ingtechnique that inherently assures perfect symmetry of each fairingsection.

A fourth important objective is to simplify the means of assembly of thefairing on the cable and to reduce the number of loose components to aminimum in the assembly operation.

BRIEF DESCRIPTION OF THE DRAWINGS We will describe our invention makingreference to the following figures in which:

FIG. I is a plan view of our fairing assembly.

FIG. 2 is an exploded perspective view of our fairing.

FIG. 3 is a sectionalized view of a group of fairings shown inrelationship to a sheave.

FIG. 4 is a fragmented perspective view of an alternate type of fairing.

FIG. 5 is a sectionalized view of a group of alternate fairings situatedon a flexed cable. Identical numerals on the several figures indicatesimilar parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a cablefairing 1 attached to a tow cable 2. The fairing comprises a nose piece3 interlocked with a pair of identical tail members 4 and 5. A link 6which connects adjacent sections of fairing as shown in FIG. 3 isretained between tail members 4 and 5 by means of projecting pin 7 bestshown in FIG. 2.

Tail members 4 and 5 are identical injection mouldings, preferably oflight weight plastic, with register pins 8 and register holes 9 soarranged that when the interfaces 10 of the two tail members 4 and 5come together, pins 8 register and insert into holes 9. A permanentconnection between members 4 and 5 may be achieved in the assemblyprocess by a snap-fit between pins 8 and holes 9 or by a solvent bondingor ultrasonic welding process.

Nose member 3 is injection moulded in'a durable and flexible materialsuch as polyurethane. A split line 11 permits the nose member to beflexed open for the purpose of wrapping it around the cable 2. Recess l2and projection 13 in nose piece 3 register with 4 and 5. Holes 16 whichpass through the necks 17 formed between split line 11 and the base ofrecess 12 register with pins 18, said pins being integrally moulded withmembers 4 and 5. This provides axial alignment between nose member 3 andtail members 4 and 5 when the three components are brought into finalregister.

Channel recess 19 in tail members 4 and 5 provides accommodation forlink 6.

Link 6 comprises a strip of durable and flexible material such aspolyurethane with two slotted holes 20, said holes being so placed anddimensioned that link 6 is not substantially thrown into tension whenthe cable is flexed to its minimum bend radius nor thrown intocompresion when the cable is straight (see FIG. 3). Flexibility of saidlink will however permit it to yield laterally to momentary misalignmentof adjacent fairing sections as said fairings pass on and off sheavesand the winch drum as indicated in FIG. 3. Chamfered entries 21 and 22to channel recess 19 provide clearance for lateral flexing of link 6.

OPERATIONAL CONSIDERATIONS Control of Compression Forces When asubmerged body is towed from a surface vessel by a tow cable at anysignificant speed, the hydrodynamic drag acting on the cable causes thecable to form a catenary curvature which .has its greatest displacementfrom the vertical at the water surface. The drag acting on the cable andfairing is a force that can be considered in two components, one actingat right angles to the cable and one acting axially down the cable.

The force that acts down the cable is a cumulative one which causes thefairing sections to crowd towards the lowest point. For this reason, thefairing sections must be designed to withstand this cumulative forcewithout undue distortion, and because of this fact, tail members 4 and 5are manufactured in a hard material to withstand this force withoutdeformation.

Function of the Interconnecting Links When the cable is submerged andtowing at any significant speed, the faired sections becomeautomatically aligned by the water flow. However, as the cable iswithdrawn from the water, usually passing over a towing sheave locatedabove the water surface, it is important that the fairing sections donot respond to wind since this may cause them to pivot around the cableat random and enter the sheave or winch drum back to front and thusbecome crushed and destroyed. Quite obviously, this situation isprevented by links 6. It may be noted that links 6 have slotted holes 20thus allowing the fairings to move apart during cable flexure withoutimposing a tensile load thereon. This is best shown in FIG. 3.

Flexibility of Fairing Nose The primary feature of our invention is thatthere is always a closed butt joint at the leading edge of the interfacebetween adjacent fairings regardless whether the cable is straight orwhen the cable is curved.

When the cable is straight the fairing nose 3 is in a substantiallyrelaxed state. When the cable flexes over a sheave 23 (FIG. 3) itslength remains constant along its neutral axis 24 and someforeshortening takes place on its concave side 25. In consequencecompression takes place at interface 26 as indicated by arrows 27 and 28and the nose material, being flexible, will yield to these forces.

There is, however, also a compression force acting upon nose 3 where itis sandwiched between the cable 2 and the sheave 23, as illustrated byarrows 29 and 30. Thus the displaced material at interface 26 will flowor squeeze into the area around the nose at the sides of the cable, asillustrated by arrows 31 and 32.

Due to the cable curvature also, the convex side of the cable will bearon the compression portion of the fairing at 33 and will cause anelongation of the nose wrap between locations 34 and 35. Thus betweenthese said locations the nose 3 will be thrown into tension and willnaturally be stretched to the required dimension. A fortuitous featureof this fairing is that the compressed material at 26 will flow into thetensioned area at 35, thus tending to reduce both the tensile andcompression stresses somewhat. Quite obviously the stresses will be at amaximum at the upper and lower edges of the fairing and they will, withthe exception of stresses illustrated by arrows 29 and 30, reduce tozero at the center of the fairing.

An alternative design of flexible nose fairing is shown in FIGS. 4 and5. Operationally this fairing is identical to the fairing depicted inFIG. 1. The structural differences are described below.

The fairing as shown in FIG. 4 comprises a flexible nose piece 36,preferably of a polyurethane material, and a pair of tail members 37 and38, preferably moulded in a rigid light weight plastic material.

In the manufacturing process an integral bond is made between nose piece36 and tail members 37 and 38.

The completed fairing may be flexed open in order to wrap it around thecable then closed and colvent bonded or ultrasonically welded at theinterface of surfaces 39 and 40.

A link 4] comprises a pair of equilateral trapezoids 42 and 43 joined bya rectangular neck 44 in the form of a planar stamping from flexiblematerial such as rubber or polyurethane (FIG. One trapezoidal end ofeach link is retained loosely in a rectangular recess 45 which is formedin the inner surface of parts 37 and 38 such that the shoulders 46 ofthe link 41 may bear against overhanging portion 47 of the recess 45 toprevent escapement of said link from said recess.

A chamfered entry 48 to said recess 45 permits said link to articulateduring bending of the cable as shown in FIG. 5.

The dimensions of link 41 and recess 45 are arranged such that the linkwill remain substantially relaxed when the cable is straight and alsowhen it is bent to its minimum radius of curvature.

The invention discussed herein has been described substantially in termsof the components used. We do not, however, wish the said invention tobe restricted to the precise arrangement of parts described since thisdisclosure is intended to explain a workable construction illustrating aconcept, and is not for the purpose of limiting the invention to anysubsequent embodiment or details thereof.

We claim:

I. An assembly of a cable and a plurality of cable fairings, eachfairing having a flexible hollow nose with said cable accommodatedtherein, and a comparatively rigid tail; said flexible nose of each saidfairings having flexibility in the longitudinal direction thereof, andeach said fairing having end surfaces from front to back thereof whichare each substantially planar; each adjacent pair of fairings beingconnected by a flexible link member; the leading edge of said fairedcable being substantially smooth and formed by the closely butted frontedges of each adjacent pair of fairings.

2. An assembly according to claim 1 wherein each said link member isflexible.

3. An assembly according to claim 1 wherein each end of each link memberis pivotable in the fairing, to which that end is connected.

4. An assembly according to claim I wherein each end of each link memberis located in the tail of the fairing, to which that end is connected.

5. An assembly according to claim 1 wherein the length of cable coveredby the nose of each fairing is substantially the same as the axiallength of said nose and said tail of said fairmg.

6. A cable fairing comprising a flexible hollow nose and a comparativelyrigid tail, said nose and said tail being made of dissimilar materials,and integrally bonded together; said cable fairing having end surfacesfrom front to back thereof said end surfaces each being substantiallyplanar; each of said nose and said tail portions being noncoextensivewith the other over the entire front to back extent of the other of saidportions; said flexible nose having flexibility in the longitudinaldirection thereof and being flexible in a forward direction away fromthe tail portion of said fairing; whereby, when the cable fairing isassociated with a cable, the cable may be reeled so that it is curved ina forward direction away from the tail of the fairing.

7. A cable fairing comprising a flexible hollow nose and a comparativelyrigid tail; said flexible nose having flexibility in the longitudinaldirection thereof; said cable fairing having end surfaces from front toback thereof, said end surfaces each being substantially planar; saidtail being made of separable portions, and said nose being split at theback side thereof and adapted to be joined on each side of said split toone of said separable portions of said tail, so as to permit entry of acable into said nose.

1. An assembly of a cable and a plurality of cable fairings, eachfairing having a flexible hollow nose with said cAble accommodatedtherein, and a comparatively rigid tail; said flexible nose of each saidfairings having flexibility in the longitudinal direction thereof, andeach said fairing having end surfaces from front to back thereof whichare each substantially planar; each adjacent pair of fairings beingconnected by a flexible link member; the leading edge of said fairedcable being substantially smooth and formed by the closely butted frontedges of each adjacent pair of fairings.
 2. An assembly according toclaim 1 wherein each said link member is flexible.
 3. An assemblyaccording to claim 1 wherein each end of each link member is pivotablein the fairing, to which that end is connected.
 4. An assembly accordingto claim 1 wherein each end of each link member is located in the tailof the fairing, to which that end is connected.
 5. An assembly accordingto claim 1 wherein the length of cable covered by the nose of eachfairing is substantially the same as the axial length of said nose andsaid tail of said fairing.
 6. A cable fairing comprising a flexiblehollow nose and a comparatively rigid tail, said nose and said tailbeing made of dissimilar materials, and integrally bonded together; saidcable fairing having end surfaces from front to back thereof said endsurfaces each being substantially planar; each of said nose and saidtail portions being noncoextensive with the other over the entire frontto back extent of the other of said portions; said flexible nose havingflexibility in the longitudinal direction thereof and being flexible ina forward direction away from the tail portion of said fairing; whereby,when the cable fairing is associated with a cable, the cable may bereeled so that it is curved in a forward direction away from the tail ofthe fairing.
 7. A cable fairing comprising a flexible hollow nose and acomparatively rigid tail; said flexible nose having flexibility in thelongitudinal direction thereof; said cable fairing having end surfacesfrom front to back thereof, said end surfaces each being substantiallyplanar; said tail being made of separable portions, and said nose beingsplit at the back side thereof and adapted to be joined on each side ofsaid split to one of said separable portions of said tail, so as topermit entry of a cable into said nose.